In many electrical systems it is common to locate a low current switch next to a high current switch on a panel. This makes sense, for example, if a particular subsystem uses a high current switch as a power control, and a low current switch as a signaling control.
Typically, it is also desirable for such switches to have the same functional “feel” so as to maintain a common tactile response for the operator. This is especially the case where the switches have a similar or matching exterior design. If these switches “feel different” during switching, the operator may mistakenly believe that one of the switches is beginning to fail, or may think that the overall fit and finish of the system is poor.
Under these aesthetic constraints, it is tempting to simply use a duplicate high current switch for a low current application, on the assumption that it will be able to handle the lower current just as well. However, the design requirements for low current switches are considerably different than for high current switches. For this reason, it is often impossible or impractical to substitute switches in this way. The challenge then is to devise an economical way to create a low current switch that has a “feel” that matches a corresponding high current switch to an acceptable degree.
In order to approach this problem, it is important to consider the design requirements of each of these types of switches. Some of the major design differences between low and high current switches relate to the effects of corrosion and switch bounce.
Low current switches are more susceptible to corrosion of contacting surfaces than switches used in high current applications, and must be designed to more aggressively minimize corrosion. Low current switches are also frequently used in applications that are sensitive to noisy signal transitions, and are best designed to minimize this effect, which is usually not an issue in high current applications. These differing requirements have an effect on the structure, “feel,” and manufacturing cost of the switch.
Using typical materials, corrosion can build up on the contacting surfaces over time, particularly in wet or corrosive environments. This corrosion forms an insulating barrier that increases resistance and interferes with the electrical contact.
High current switches can tolerate a certain degree of tarnish or corrosion on contacting surfaces because the high current is sufficient to “punch through” the corrosion. For a given switch design, the minimum current required to break through the expected corrosion resistance is commonly known as the “wetting current”.
Wetting current is the lowest current that an electronic circuit can operate under. Below the wetting current, current will not flow at all. However in low current applications, the current is usually below the wetting current for a typical high current switch. This means that the current in the system may be insufficient to “punch through” the corrosion that forms on the contacting surfaces, eventually causing the switch to fail.
In order to address this issue for low current applications, the contact points and certain types of conducting joints (such as pivot or bearing surfaces) within the switch must be made from or coated with a minimally corroding substance in order to prevent the gradual buildup of tarnish or other corrosion.
Gold plating is a standard choice for providing a highly conductive and non-corroding surface. However gold is very expensive, with costs increasing dramatically in recent years.
Because it is so expensive, the gold plating used in switches is often extremely thin in order to reduce costs. If the coating is made too thin, however, this can negatively impact reliability by making the switch overly susceptible to wear-through and eventual corrosion.
This means that adapting a typical pivoting high-current contact switch for use in low current applications by simply adding gold plating can represent a significant increase in materials cost, and may not be sufficient to produce a low current switch having a sufficiently long life.
Because of these issues, a low current switch based on a high current design may need to be structurally redesigned to minimize the amount of gold that is required. However, these modifications have the potential to change the feel of the switch.
Low current applications are also often susceptible to switch contact bounce. This is particularly true in digital circuits where an unambiguous transition between signaling levels can be important for proper operation.
Switch bounce occurs when the contacts of the switch open and close. As the contacts come together, the mass, inertia, and surface characteristics of the contact cause the contacts to “bounce” or rapidly open and close several times before coming to rest in the closed position. A similar effect can occur as the contacts separate and before they come to rest in the open position.
Because high current circuits are usually not especially sensitive to noisy switching signals, such switches are frequently designed to handle the desired current load without regard to bounce. Thus, the structures of a high current switch may produce signals that are too noisy for some low-current digital signaling applications.
In order to use such a switch for low current applications, the circuit being switched may need to be “debounced” using additional components in order to create a reliable signal. However debouncing circuitry requires additional cost to manufacture. In some high speed digital applications a delay is introduced to adequately debounce a switch designed for high current use.
Because of these issues, a low current switch based on a high current design may need to be structurally redesigned to minimize switch bounce. But as with a redesign to reduce corrosion with a minimum of gold plating, these modifications have the potential to alter the feel of the switch.
It is therefore desired to provide a low current switch that addresses these deficiencies.